The moment I have been waiting for since starting the course has finally arrived- learning how to service a mechanical chronograph. We get the chance to service a number of chronograph movements during this stage of the WOSTEP course which allows us to experience the different types of mechanisms used. The purpose of the next few weeks is to gain a greater understanding of the most popular chronograph systems and learn how to service them to a high standard. Our first port of call would be the legendary workhorse chronograph, the ETA Valjoux 7750, probably the most popular chronograph in use today and one that is used by a wide spectrum of brands across all areas of the market.

I thought that I would start with a brief history of the mechanical chronograph and of course, the story of the 7750 itself. So here goes…

The majority of chronographs available in the 1960s used a mechanism called a column wheel to start, stop and reset there stopwatch functions. It was renowned for its precision and snappy response but the column wheel took a lot of skill to produce and was widely recognised as a high achievement for a watchmaker. The design didn’t lend itself well to mass production and inevitably commanded a large price tag, as a result it was an easy target for the large quantities of cheap quartz watches coming out of Japan at this time. The mechanical chronograph was in decline…

In 1973, Ébauches SA, the Vallée de Joux movement maker, decided to reengineer one of their old hand wind chronograph movements and as a result the Valjoux 7750 was born. The new movement incorporated a cam and lever system instead of the more traditional column wheel that usually controlled the chronograph stop, start, and reset functions. The cam system, also known as a coulisse lever system, drives the stopwatch functions through a series of levers that push an oblong cam back and forth, enabling the starting, stopping and resetting of the chronograph. The coulisse system was cheaper to manufacture than a column wheel because its parts were manufactured by stamping. The movement was not as refined as its column wheel rivals but its relative crudeness was made up for by its robustness and reliability.

During the mid-70s, quartz watches ruled the market and there wasn’t a large demand for mechanical chronographs, the 7750 lay largely dormant. In 1975, the Valjoux 7750 fell victim to the quartz crisis that devastated the Swiss watch industry and was assigned to the scrap heap. The management of Valjoux ordered the destruction of all 7750 movements in stock and all of the tooling used to make them. The man behind the development of the 7750 protested but his arguments were ignored so he took matters into his own hands. He carefully packed up the tooling and movements for safekeeping, and hoped that the industry would one day recover and offer the 7750 a brighter future.

The industry took eight long years to recover and it was 1983 when mechanical watches had begun their dramatic comeback. Thanks to its creators foresight to rescue the tooling used to make it, the Valjoux 7750 returned to the scene. Valjoux SA had by now become part of ETA, the prominent movement manufacturer now part of the Swatch Group empire. The relaunch proved a success and modifications of the 7750 soon began to appear. The 7751, which was considerably more complex, had a full calendar, with a centrally mounted hand for the date, windows for both the day of the week and the month, plus a moon-phase at 6 o’clock appeared in the mid 1980’s. The 7750 went on to become a huge success in its various guises and can be credited with bringing mechanical chronographs to the masses.

In its most popular modern specification, the 7750 contains 25 jewels, beats at 28,800 BPH and has a respectable power reserve of 46hrs. The movement is widely recognised as being extremely versatile and is commonly used as a base for further complications such as a rattrapante function. The reason we are starting are chronograph training with this movement is mainly because of its widespread use but also because its a relatively easy movement to work on. As the owner of a couple of 7750 powered watches myself, I was keen to find out how it worked and decide whether or not it lived up to its reputation.

We started off with this…

The plan was to end up with this after a step by step reassembly…

Unfortunately this was one of those occasions where I couldn’t capture every single step due to the need to keep up to speed with the assembly so the pictures are a mixture of 7750 and 7751 which are identical with the exception of the calendar. Unfortunately I only managed to take pictures up to a certain point but you can find the rest at the ETA SwissLab site.

The main plate ready for assembly…

In goes the barrel…

Followed by the balance stop and train wheels…

The train bridge can now be put in place…

The ratchet wheel can now be fitted- note the click spring is part of the bridge rather than being a separate part like most ETA products. Two spots of lubrication are also applied in preparation for fitting the crown wheel…

The crown wheel can be placed in position and two spots of lubrication are applied in preparation for fitting the crown wheel core…

The crown wheel core is then fitted…

The movement is then turned over so they assembly of the keyless work can begin with the winding stem, sliding and winding pinions…

Followed by the yoke spring…

Then the rocking bar, setting lever and yoke…

And finally the setting wheel, intermediate setting wheel and the setting lever jumper…

The train wheel jewels can be oiled on both sides movement and the pallets are put in next along with their bridge…

The balance and its shock settings are last to go in…

At this point the escapement is oiled and the amplitude of the movement is checked on a timing machine. Its important to check the amplitude is healthy before fitting the chronograph mechanism because if its found not to be after assembling everything, the whole movement would have to disassembled and cleaned again. Valuable time could be lost for the sake of skipping a simple check.

The clutch and the automatic reverser wheel are put into position next…

Once the movement is serviced, there are a number of things to check on the chronograph mechanism and generally some adjustments will need to be made to ensure it functions correctly. The first check is to ensure the minute counter jumper spring is aligned with the edge of the chronograph bridge.

The second check is to ensure the depth of the teeth on the oscillating pinion are engaged with the teeth on the chronograph wheel at approximately two thirds of their length. The depth of engagement is adjusted by an eccentric screw. The third check is to ensure the end of the minute counter jumper spring is engaged with two teeth on the minute counter wheel. This is adjusted by turning another eccentric screw. The final check is to ensure the chronograph wheel jumper turns the minute counter wheel after 1 minute, for this to happen, the tooth of minute counter wheel should hit the straight section of the spring approximately 1/2 way across. To adjust this, the spring is bent accordingly with a pair of tweezers.

This was the timing machine reading for the first 7750 I serviced, these figures are without the chronograph running, there was a loss of around 15-20 degrees in amplitude with it running which is perfectly acceptable. The Delta (D) was quite high on this movement due to the hairspring not being perfectly centred which is shown by the difference between the 9H position and the 3H position. I was pretty pleased with this though considering it was my was attempt at servcing a chronograph.

Timing machine results…

After completing the 7750 which was a movement only, we got the chance to work on a complete ETA training watch which housed the more complex 7751. This movement is identical to the 7750 with the exception of the calendar work which is a lot more complex due to it housing triple date and moon phase complications along with a 24hr indicator. This in itself presented more of a challenge when it came to assembling the movement but even more so when fitting the hands. I have never had to fit some many hands to one watch before, the hands also have to be fitted on correctly so that each complication changes at the correct time. This was quite a challenge and suffice to say, the hands were fitted and refitted on a number of occasions before they were correct.

The 7751 training watch…

Here are my first set of timing results…

The loss of amplitude from one position to another was pretty good but the overall amplitude was pretty low on both accounts. The solution was to reduce the depth of the pallet stones on the escape wheel slightly. Once I had done this, I had a respectable average of 293 degrees without the chronograph running and 270 degrees with it running. The 7751 with all its complications was definitely more challenging than the basic 7750, especially fitting the hands which was no easy task!

So what do I think of the 7750? Well, bearing in mind I had never worked on a chronograph before this one, I found it fairly easy to work on and very robustly made. In fact aside from the extra components and adjustments, it was just as easy to service as a basic mechanical watch. This was quite a surprise to me as I had always imagined chronographs to be much more difficult to service and probably a lot troublesome. Its difficult for me to comment on the cam system used by the 7750 at this point in comparison to the column wheel system as I’m yet to service one.

What I will say about the cam system is that it has a positive action and the cam along with its other components seem extremely robust. The cam system is simple to understand and easy to assemble. I guess in this respect the 7750 meets its brief, it does what a column wheel chronograph does but allows easier and cheaper manufacture which in turn provides an affordable mechanical chronograph. The whole movement seems designed to last, there are very few weak components to potentially fail and I guess thats why its proved to be so reliable and robust over the years. So in conclusion, in my opinion and with my limited chronograph experience so far, I feel that the 7750 is well deserving of its positive reputation.

The next movement we are due to service is the Longines L688, which is based on the 7750 but has been modified to include a column wheel. Hopefully I will be able to give an opinion on the cam system versus column wheel system once I’ve had the opportunity to service both. Thanks for reading if you got this far and keep an eye out for my next post.